 /*******************************************************************************
  * Copyright (c) 2005 IBM Corporation and others.
  * All rights reserved. This program and the accompanying materials
  * are made available under the terms of the Eclipse Public License v1.0
  * which accompanies this distribution, and is available at
  * http://www.eclipse.org/legal/epl-v10.html
  *
  * Contributors:
  * IBM Corporation - initial API and implementation
  *******************************************************************************/
 package org.eclipse.core.commands.operations;

 /**
  * <p>
  * This class is used to maintain the instance of the operation history that
  * should be used by classes that access the undo or redo history and add
  * undoable operations to the history.
  *
  * <p>
  * It is intended that an application can create an operation history appropriate
  * for its needs and set it into this class. Otherwise, a default operation history
  * will be created. The operation history may only be set one time. All classes that
  * access an operations history use this class to obtain the correct instance.
  *
  * @since 3.1
  */
 public final class OperationHistoryFactory {

     private static IOperationHistory operationHistory;

     /**
      * Return the operation history to be used for managing undoable operations.
      *
      * @return the operation history to be used for executing, undoing, and
      * redoing operations.
      */
     public static IOperationHistory getOperationHistory() {
         if (operationHistory == null) {
             operationHistory = new DefaultOperationHistory();
         }
         return operationHistory;
     }

     /**
      * Set the operation history to be used for managing undoable operations.
      * This method may only be called one time, and must be called before any
      * request to get the history. Attempts to set the operation history will
      * be ignored after it has been already set, or after a default one has
      * been created.
      *
      * @param history
      * the operation history to be used for executing, undoing, and
      * redoing operations.
      */
     public static void setOperationHistory(IOperationHistory history) {
         // If one has already been set or created, ignore this request.
 if (operationHistory == null) {
             operationHistory = history;
         }
     }
     
     private OperationHistoryFactory() {
         // may not be instantiated
 }

 }

